1. Field of the Invention
The present invention relates to sheet separating and feeding device, and more particularly, to apparatus for successively separating the top sheets from a stack of sheets and for feeding the successively separated sheets from the stack. 2. Prior Art
The prior art abounds with numerous devices for separating sheets from a stack and feeding the separated sheets. By way of example, U.S. Pat. No. 3,008,709 to Buslik describes a wave generator (sometimes called a combing wheel) for separating sheets from a stack. In the Buslik device, a wave generator is disposed to rotate in a plane parallel to a stack of sheets. The wave generator includes a disc fixedly attached to a rotating shaft. A plurality of free rolling balls are affixed to the disc. The rotating shaft is raised and lowered under the control of a spring and solenoid. The direction of shaft motion is generally perpendicular to the stack. In operation, the rotating disc and free rolling balls are lowered to contact the topmost sheet in the stack. The rotary motion is imparted to the stack and sheets are shingled or separated in a fan-like manner until the topmost sheet is positioned for further feeding.
U.S. Pat. No. 4,165,870 to Fallon et al. describes another prior art rotary shingler device. In the Fallon device, a metal disc is rigidly mounted to a shaft. A plurality of free-rolling wheels or rollers are mounted to the periphery of the disc. The shaft is tiltable about an axis substantially perpendicular to a stack of sheets. A drive means is coupled to the shaft and rotates the disc in a plane substantially parallel to the stack. A sheet feeding assembly including a backup surface and a rotating roller is disposed to form a feed nip relative to the stack. In operation, the shaft is tilted so that one set of the rollers contacts the topmost sheet in the stack. The shaft is then rotated and the sheet is shingled in a linear path away from the feed nip. The shaft is tilted in another direction and another set of rollers contacts the sheet shingling the sheet in the opposite direction into the feed nip.
U.S. Pat. No. 3,583,697 to Tippy is yet another example of the prior art sheet separating and sheet feeding devices. In the Tippy device, a paper stack is disposed in a tray so that the leading edge of the stack forms an angle with an axis of a pair of sheet feed rollers disposed relative to said stack. A single roller is mounted to a rotating shaft. The shaft is mounted above the stack with the periphery of the roller being in driving engagement with the topmost sheet in the stack. The geometric configuration between the elements of the sheet separating and sheet feeding devices are such that the shaft runs in a general direction parallel to the axis of the feed rollers while the single roller is positioned off-center of the stack. As the single roller rotates and is brought into contact with the topmost sheet, the sheet is rotated off the stack with its leading edge in parallel alignment with the feed rollers.
IBM.sup.R Technical Disclosure Bulletin (TDB) Vol. 21, No. 12, May 1979 (pages 4751-4752) describes a lightweight modular sheet feed and delivery apparatus for attachment to a printer. In the article, two roll wave separators of the type described in the above Fallon et al. patent are disposed for shingling sheets from two removable cassette-type hoppers. Each hopper contains different sizes and/or types of paper. As sheets are shingled from each of the respective hoppers, a pair of feed rollers feeds the shingled sheets towards a common channel. Sensors are disposed relative to each hopper. The sensor senses the leading edge of a shingled sheet and initiates a signal to deactivate the appropriate roll wave separator.
IBM.sup.R TDB Vol. 21, No. 12, May 1979 (page 4747) describes a roll wave separator of the type described in the Fallon et al. patent. In the article, the roll wave separator is slidingly connected to a shaft. The shaft is disposed relative to a stack of sheets with the roll wave separator floatingly engaged to the topmost sheet in the stack. As sheets are fed from the stack, the roll wave separator adjusts to the stack height, thus eliminating the need for a sheet elevator.
In IBM.sup.R TDB Vol. 21, No. 12, May 1979 (pages 4748-4749) describes a rotating roll wave separator of the type described in the Fallon et al. patent. The roll wave separator is disposed at the center of a stack of sheets. By contacting the stack with the roll wave separator and simultaneously applying a slight force and rotating said wave separator, a sheet is rotated from the stack.
In IBM.sup.R TDB Vol. 22, No. 6, November 1979 (pages 2169-2170) shows a picker roller paper feed device with paper depressor element. The device includes a plurality of free-rolling small wheels disposed about the periphery of a disc. When the disc is lowered into contact with a stack, the lower surface of the disc serves as a paper depressor while the free-rolling wheels dislodge a sheet from the stack along a linear path.
IBM.sup.R TDB Vol. 20, No. 6, November 1977 (pages 2117-2118) describes a combing wheel wave generator coacting with a variable force brake to feed a single sheet from a stack. The combing wheel wave generator is disposed at the front of the stack while the variable force brake is positioned at the rear of said stack. A solenoid controls the brake so that its force on the stack is decreased when the combing wheel is in contact with the stack.
U.S. Pat. No. 3,989,237 to Goff describes a variable force sheet feeding device wherein a variable force means applies a horizontal force to the topmost sheet on a stack. The force is increased until the sheet buckles. As the buckle is sensed, the feed means changes the direction in which the force is applied and the sheet is fed along a linear path from the stack. The process of buckling the sheet in one direction and feeding said sheet in the opposite direction, is a reliable method to feed paper of varying types and/or weights.
U.S. Pat. No. 3,861,671 describes a document handling device wherein a bail bar is utilized to provide a normal force on a stack of sheets to enable a feed roll therebeneath to positively feed a single document or a number of documents from the stack beneath the bail bar. Bail bar pressure on the feed roll is released after initial feeding of each document to allow multifeed documents to be returned to the document stack by a suitable document return mechanism.
U.S. Pat. No. 3,869,116 describes a card feed device having a magnetic force application mechanism to apply a normal force to a stack of cards. A feed roll disposed beneath the stack feeds card forms from the stack.
U.S. Pat. No. 798,857 describes a variable weight mechanism which is applied to the top of a stack to enable feeding of sheets from the bottom.
Although the above prior art wave generator sheet separating devices work satisfactory for their intended purpose, there appears to be a lack of control between the devices and sheets in the stack. The lack of control results in double sheet feed from the stack, inconsistent positioning of the sheet relative to a subsequent sheet feed apparatus and relatively long shingle time. It is believed that the lack of control is caused by the fact that the stack is not perfectly flat, therefore, the plane of the paper is not parallel to the plane of the wave generator sheet separating devices. The nonparallelism between the stack and sheet separating device is usually brought about by environmental conditions. For example, humid conditions tend to cause the paper to raise and buckle. Attempts to control the environment tend to be costly and nonacceptable.
Another drawback associated with the above prior art devices is the inability to handle a wide range of paper types and weights. The Goff patent solves the problem by buckling the sheet and then feeding in a direction opposite to the buckle. Although this approach works well for low speed devices, it is unacceptable for high speed devices. Usually the time used to buckle and then feed a sheet is greater than the time allotted to feed a sheet in a high performance device. This is particularly true in machines such as convenience copiers wherein a sheet must be delivered to transfer station within a relatively short time so that a developed image can be transferred to the sheet.